Most older Raleigh bicycles used a unique 26 tpi threading on their bottom bracket shells, which means replacing the BB on one can prove extremely challenging.  Phil Wood has provided one of the few solutions to this issue: bottom bracket cups threaded to the old Raleigh standard.  Note: will also require a Phil Wood manufactured spindle (Sheldon Brown suggests that a longer JIS spindle will work) and bearings.  Will work for a Raleigh with a 71mm BB shell, but probably not a 76mm.  Most likely this will be your only option.